The safety and convenience of hydrogen tank refueling are recognized as important considerations in determining the ultimate success of hydrogen fueled vehicles in the marketplace. Under current safety guidelines, the refueling of compressed hydrogen tanks are to be conducted in a manner that prevents the tank from overheating (temperatures exceeding 85° C.) during refueling and/or from overfilling the tank to a point at which the pressure could exceed 125% of normal working pressure (NWP) at any time. Because of the number of unknown parameters associated with conventional hydrogen tank refueling procedures, the refueling operations tend to be somewhat conservative, thereby trading performance and efficiency, particularly with respect to end of fill density (SOC %) and/or unnecessary levels of pre-cooling, for an increased safety margin. A SOC of 100%, for example, corresponds to a tank at NWP and 15° C.
This tradeoff is especially significant in non-communication fueling operations in which the parametric assumptions are even more conservative. Because the hydrogen station does not have information about the tank that it is filling, very conservative assumptions need to be made for the system in order to encompass the range of possible tank configurations and initial tank conditions to avoid exceeding the system safety limits. In SAE TIR J2601, the disclosure of which is incorporated herein by reference, in its entirety, these conservative assumptions are incorporated into a series of lookup tables for hydrogen tank filling. Working from parameters including the tank volume, starting pressure, ambient temperature and station pre-cooling set point, the lookup tables are then used for determining a pressure ramp rate and final target pressure. While application of these lookup tables tends to provide for safe refilling under virtually all conditions and for virtually all tank systems, given the conservative nature of the associated assumptions, the resulting hydrogen tank filling operation may take longer, achieve lower final fill pressures and/or require lower hydrogen station pre-cooling temperatures than necessary to fill a particular tank system.
An additional limitation of the refilling procedures defined by SAE TIR J2601 is the lack of any method or procedure for a hydrogen tank filling station to compensate or adjust for situations in which its actual operating conditions fall outside of the allowed tolerances. For example, if the pre-cooling temperature is above the design set point as the result of multiple consecutive refills, the lookup tables defined in SAE TIR J2601 cannot be used. Efforts to avoid this out of specification condition can lead to an overdesigned hydrogen tank filling station (excessive cooling for ensuring that the pre-cooling target temperature is maintained), thereby driving up station cost.
Conversely, failing to ensure that the pre-cooling target temperature is maintained can inconvenience customers that are unable to refill their tanks in a timely manner (as a result of delays waiting for the pre-cooling temperature to come into specification), thereby reducing customer satisfaction, station revenue and/or repeat business. Further, operating a station with a constant pre-cooling temperature regardless of current ambient conditions results in excessive energy usage and reduced well-to-wheel energy efficiency. In order to reduce energy use, a hydrogen tank filling station should be operated at the highest possible pre-cooling temperature that provides both customer-acceptable refueling times and a satisfactory safety margin.